Such rotary-screen units can be used to imprint a given web section successively with different color components of the recurrent pattern. If the length of a section exceeds the circumference of the screens, several screens of consecutive units may be grouped together to print different parts of the pattern in the same color. Thus, each group may consist of a first-stage unit and a second-stage unit respectively printing the front half and the rear half of the pattern; in this case the first-stage units are the odd-numbered ones and the second-stage units are the even-numbered ones as counted from the upstream end of the array.
Because of unavoidable deviations of the actual length and spacing of successive web sections from the specified dimensions, cumulative errors would result if the angular positions of the screens were not adjusted from time to time in the course of a printing run. To compensate for such errors, it has already been proposed (see U.S. Pat. No. 3,152,542) to provide automatic means for individually adjusting each printing screen in response to output signals from a pair of associated sensors, one of them being located near the screen itself while the other lies upstream of the screen adjacent the web path and is spaced from the point of contact between the screen and the web, i.e. from the nadir of the screen, by a certain distance determined by the length of the pattern. If the screen and the web are properly correlated, the two sensors simultaneously detect respective marks on the screen and the web.